As a light source for a projector, a short-arc high-pressure discharge lamp (hereinafter referred to simply as “lamp”) is widely used, for example. The lamp is a light source that is close to a point light source, and has a high luminance and a high color rendering index. For example, the lamp is built into a reflector so as to form a lamp unit, the reflector having a concave reflective surface, and a longitudinal central axis of the lamp substantially coinciding with an optical axis of the reflector. The lamp unit is built into a projector and used.
An arc tube that mainly constitutes the lamp has a casing made of quartz glass. The arc tube includes a light-emitting part at a center thereof and seal parts adjacent to the light-emitting part, which extend outwards from each side of the light-emitting part. A pair of electrodes is disposed in the light-emitting part. In addition, a discharge space is formed within the light-emitting part.
The lamp is lighted under a high load in order to achieve the high luminance and the high color rendering index, as described above. As a result, the light-emitting part, especially an upper part thereof (an upper part of the discharge space) becomes extremely high temperature while the lamp is lighted. The quartz glass constituting the part crystallizes, and thereby cloudiness called devitrification occurs. In consequence, this part might be eventually damaged.
In addition, in the case where the temperature difference occurred in the light-emitting part is great, when tungsten that constitutes the electrodes diffuses during lighting, the tungsten blackens after attaching to a lower temperature part in the arc tube (e.g., a root part of the electrodes). As a result, this might cause lower luminance.
Because of this, it is preferable that the temperature in the arc tube, specifically in the light-emitting part should not overly rise, and that temperature distribution in the light-emitting part should be uniformed during lighting. Especially, in recent years, requirements for size reduction and high-power output of a projector are becoming higher. This leads to requirements for a small size arc tube and high-watt arc tube, and load on an arc tube is increasing. As a result, the problem above is becoming prominent.
In response, conventionally, there has been proposed an approach to solve the above problem by rotating an arc tube.
For example, there has been proposed an art of periodically rotating only an arc tube by a rotation unit connected to the arc tube (Patent Literature 1, for example), or an art of rotating not only an arc tube (a high-pressure discharge lamp) but also a reflector (a reflection mirror) integrated with the lamp when rotating the lamp by a rotation mechanism connected to the lamp (Patent Literature 2, for example). In addition, there has been proposed an art of providing a lamp unit, which includes an arc tube (a high-pressure discharge lamp) and a reflector (a reflection mirror), intermittently rotating the lamp unit not only in one direction but also in the opposite direction by a rotation mechanism, and reversing a rotational direction during the rotation (Patent Literature 3, for example).